Back contact solar cell and fabrication method thereof

ABSTRACT

The present invention discloses a back contact solar cell. The back contact solar cell includes a semiconductor substrate having a front surface and a rear surface; a first conductive type semiconductor region having a first conductive type and a second conductive type semiconductor region having a second conductive type at an interval on the rear surface of the semiconductor substrate. Furthermore, the rear surface of the semiconductor substrate has a texturing structure at the interval between the first conductive type semiconductor region and the second conductive type semiconductor region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/812,910 filed on Jan. 21, 2011, which is the national phaseof PCT International Application No. PCT/KR2008/006685 filed on Nov. 13,2008, which claims the benefit of Korean Patent Application No.10-2008-0016725 filed on Feb. 25, 2008. The entire contents of all ofthe above applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fabrication method of a back contactsolar cell, and in particular to a fabrication method of a back contactsolar cell capable of simplifying a fabrication process thereof and alsoreducing fabrication costs thereof by forming a pattern for formation ofa p-type semiconductor region and an n-type semiconductor region on arear surface of the back contact solar cell by a laser method to allow acomplicated process such as photolithography to be omitted.

2. Discussion of the Related Art

Recently, due to problems such as a rapid rise in oil prices, anenvironmental problem of the earth, exhaustion of fossil energy, wastetreatment in nuclear power generation, position selection according toconstruction of a new power plant, etc., interest in renewable energyhas increased and among others, research and development for a solarcell, which is a pollution-free energy source, has actively beenprogressed.

The solar cell, which is a device converting light energy intoelectrical energy using a photovoltaic effect, is divided into a siliconsolar cell, a thin film solar cell, a dye sensitized solar cell, and anorganic polymer solar cell, etc., according to type and configuration ofmaterials. Such a solar cell is independently used as a main powersupply in an electronic watch, a radio, a manless lighthouse, asatellite, a rocket, etc., and is also used as an auxiliary power supplyin connection to a system of a commercial AC power supply. Recently, dueto increase in the necessity for alternative energy, interest in thesolar cell has increased.

A structure of a crystalline silicon solar cell firstly used among thesesolar cells and being also used to some degree in the current solar cellmarket is shown in FIG. 1. Hereinafter, a fabrication method of aconventional crystalline silicon solar cell will be described withreference to FIG. 1.

First, a texturing structure for minimizing reflectance of incidentlight is formed on a surface of a p-type silicon substrate 110.Thereafter, an n-type material such as POCl₃ is thermally diffused ontoa whole surface of the substrate 110 to form an n-type semiconductorlayer 120, thereby forming a p-n junction.

Next, an anti-reflection coating 130 for minimizing the reflectance isformed on a surface, which is a light-receiving face in the substrate110, that is, a surface to which solar light is incident. Thereafter,rear electrodes 140 such as an aluminum electrode, etc. are formed on arear surface of the substrate 110 and at the same time, a rear electricfield layer 150 is formed by thermally processing the rear surface ofthe n-type semiconductor layer 120.

Finally, front electrodes 160 penetrating the anti-reflection coating130 to reach to the n-type semiconductor layer 120 are formed tocomplete the crystalline silicon solar cell.

However, such a crystalline silicon solar cell has a disadvantage thatsince there are the front electrodes 160 formed in a metal finger lineform on the surface to which the solar light is incident, that is, thelight-receiving face side, it is impossible to avoid shadowing.

More specifically, the metal finger line is formed in a form protrudedon the light-receiving face so that the shadowing is generated, whereinthe shadowing reduces an area to which the solar light may be incident,thereby having a bad effect on efficiency of the solar cell.

For this reason, the crystalline silicon solar cell has a problem thatit is difficult to generate a high efficiency of 18% or more. In orderto solve the problem, a back contact solar cell has been designed.

FIGS. 2 to 10 show a fabrication process of a conventional back contactsolar cell. Hereinafter, the fabrication process of the conventionalback contact solar cell will schematically be described with referenceto FIGS. 2 to 10.

First, as shown in FIG. 2, a cut surface of an n-type silicon substrate210 cut in a predetermined size is partially etched and prepared.Thereafter, as shown in FIG. 3, a texturing structure for minimizingreflectance of incident light is formed on at least one of an uppersurface or a lower surface of the silicon substrate 210 using a basesolution, etc.

Thereafter, as shown in FIG. 4, a rear surface of the substrate 210 isfinely polished for planarization.

Next, as shown in FIG. 5, p-type dopant sources 230 and n-type dopantsources 250 are printed on the rear surface of the substrate 210 using ascreen printer, etc. Thereafter, as shown in FIG. 6, the p-type dopantsources 230 and the n-type dopant sources 250 are diffused to form ann-type region and a p-type region, respectively.

Next, as shown FIGS. 7 and 8, thermal oxide layers 261 and 263 forsurface passivation and an anti-reflection coating 270 made of siliconnitride SiN_(x), etc., are formed on front and rear surfaces of thesubstrate 210.

Next, as shown in FIG. 9, the oxide layer 263 formed on the rear surfaceof the substrate 210 is partially removed. This process is to form rearelectrodes, that is, electrodes electrically contacting the p-typeregion and the n-type region. In this process, a photolithographyprocess is used in order to remove the oxide layer 263.

More specifically, a photoresist is applied on the oxide layer 263, anda mask having a pattern corresponding to a region of the oxide layer 263to be removed for formation of the rear electrodes is applied thereonand is exposed so that the oxide layer 263 is selectively removed.

After partially removing the oxide layer 263 through this process, therear electrodes 271 and 273 electrically connected to the p-type regionand the n-type region are formed, as shown in FIG. 10, to complete theback contact solar cell.

The photolithography process is requisite for the fabrication process ofsuch a conventional contact solar cell. That is, the rear electrodeshave been formed in order to solve the problem of efficiencydeterioration due to the shadowing of the conventional crystallinesilicon solar cell; however, it is requisite to partially remove theoxide layer 263 in order to form the rear electrodes and to this end,the photolithography process should be performed.

Such a photolithography process is very complicated as well as causes anincrease in process time and fabrication costs, thereby loweringfabrication efficiency of the back contact solar cell.

Therefore, it is very required to develop technology capable of solvingthe problem of a low efficiency due to the shadowing and at the sametime, fabricating a back contact solar cell only by a simplifiedprocess.

SUMMARY OF THE INVENTION

The present invention has been proposed in order to solve the problem inthe prior art as described above. It is an object of the presentinvention to provide a fabrication method of a back contact solar cellcapable of obtaining effects such as rise in efficiency due tominimization of a shadowing effect, which is an original advantage ofthe back contact solar cell, reduction in recombination of electron-holepairs due to the rear surface passivation, etc., and at the same time,accomplishing a simplified fabrication process and a low fabricationcosts, by using a laser method instead of a complicated process such asphotolithography, etc., conventionally used to form a pattern forformation of a p-type semiconductor region and an n-type semiconductorregion comprised in a rear surface of the back contact solar cell.

In order to accomplish the above object, the present invention providesa fabrication method of a back contact solar cell having a plurality ofdifferent conductive type semiconductor regions on a rear surface of afirst conductive type semiconductor substrate comprising the steps of:forming oxide layers on front and rear surfaces of the substrate; andremoving the oxide layer by irradiating laser light to the oxide layerformed on the rear surface of the substrate at predetermined intervalsto form a pattern of the different conductive type semiconductorregions.

The first conductive type semiconductor substrate may be changedaccording to a conductive type of dopant, and is not necessarily limitedto a specific conductive type, but preferably may be a p-type siliconsubstrate.

In the present invention, the fabrication method of the back contactsolar cell may further comprise the step of alternatively forming afirst conductive type semiconductor region and a second conductive typesemiconductor region by diffusing different conductive type dopants intothe pattern formed at the predetermined intervals. The first conductivetype semiconductor region and the second conductive type semiconductorregion may be an n-type (n+) semiconductor region and a p-type (p+)semiconductor region, respectively.

A method forming the n-type semiconductor region is not specificallylimited, but may be any one n-type dopant doping method selected from agroup consisting of an ion implantation method, a thermal diffusionmethod, and a phosphorous oxychloride (POCl₃) diffusion method. Morespecifically, the n-type semiconductor region may be formed by a methodinserting a p-type substrate into a high-temperature furnace andinjecting an n-type impurity generation gas at a high concentrationthereinto while heating temperature to 800 or 900° C., wherein then-type impurity generation gas may preferably be phosphorous oxychloride(POCl₃).

Also, a method forming the p-type semiconductor region is notspecifically limited, but the p-type semiconductor region may be formedby a screen printing method of a p-type dopant material.

The fabrication method of the back contact solar cell of the presentinvention may further comprise the step of forming metal electrodes oneach of the first conductive type semiconductor region and the secondconductive type semiconductor region after forming the first conductivetype semiconductor region and the second conductive type semiconductorregion.

The metal electrode may be formed by, but not necessarily limited to, ascreen printing method.

In the fabrication method of the back contact solar cell of the presentinvention, a laser source used to irradiate the laser light is notspecifically limited, but preferably may be a green laser source or anNd/YAG laser source.

The fabrication method of the back contact solar cell of the presentinvention may further comprise the step of forming an anti-reflectioncoating on an upper surface of the oxide layer formed on the frontsurface of the substrate.

Also, the front and rear surfaces of the first conductive semiconductorsubstrate may have a texturing structure.

A fabrication method of a back contact solar cell having a plurality ofdifferent conductive type semiconductor regions on a rear surface of ap-type semiconductor substrate according to one embodiment of thepresent invention comprises the steps of: forming oxide layers on frontand rear surfaces of the substrate; forming n-type semiconductor regionsin a pattern in which the oxide layer is removed by irradiating laserlight to the oxide layer formed on the rear surface of the substrate atpredetermined intervals; forming p-type semiconductor regions in apattern in which the oxide layer is removed by irradiating laser lightto the oxide layer between the n-type semiconductor regions atpredetermined intervals from the n-type semiconductor region; andforming metal electrodes on each of the n-type semiconductor region andthe p-type semiconductor region.

A back contact solar cell of the present invention comprises: a firstconductive type semiconductor substrate having a front surface and arear surface of a texturing structure; an oxide layer formed on thefront surface of the substrate; at least one first conductive typesemiconductor region and second conductive type semiconductor regionalternatively formed at predetermined intervals on the rear surface ofthe substrate; an oxide layer formed on the remaining rear surface ofthe substrate except for the first conductive type semiconductor regionand the second conductive type semiconductor region; and electrodesformed on each of the first conductive type semiconductor region and thesecond conductive type semiconductor region.

The first conductive type semiconductor substrate may be a p-typesilicon substrate, and the first conductive type semiconductor regionand the second conductive type semiconductor region may be an n-type(n+) semiconductor region and a p-type (P+) semiconductor region,respectively.

A resultant formed by oxidation will be sufficient as the oxide layer;however, the oxide layer preferably is silicon oxide (SiO₂) formed byrapid thermal oxidation (RTO). Also, the back contact solar cell mayfurther comprise an anti-reflection coating on an upper surface of theoxide layer formed on the front surface of the substrate.

According to the fabrication method of the back contact solar cell ofthe present invention, it is possible to simplify a fabrication process,reduce fabrication time, and reduce fabrication costs by using the lasermethod instead of photolithography in forming the pattern for formationof the p-type semiconductor region and the n-type semiconductor region.

Also, it is possible to obtain effects such as a rise in efficiency dueto minimization of the shadowing effect, which is an original advantageof the back contact solar cell, a reduction in recombination ofelectron-hole pairs due to rear surface passivation, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view showing a structure of a conventionalcrystalline silicon solar cell;

FIGS. 2 to 10 are schematic views showing a fabrication process of aconventional back contact solar cell; and

FIGS. 11 to 17 are schematic views showing a fabrication process of aback contact solar cell according to one embodiment of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS

FIGS. 11 to 17 are schematic views showing a fabrication process of aback contact solar cell according to one embodiment of the presentinvention. Hereinafter, the fabrication process of the back contactsolar cell according to the present invention will be described withreference to FIGS. 11 to 17.

First, as shown in FIG. 11, a texturing structure is formed on at leastone of a front surface and a rear surface of a p-type silicon substrate310. The texturing structure may usually be formed in a pyramid shape,etc, and performs a function of reflecting solar light incident to thesolar cell so that maximum light may be absorbed into an inside of thesolar cell, thereby raising efficiency of the solar cell.

The texturing structure may be formed by etching using a known etchingmethod. As an example, the texturing structure may be formed byimmersing the silicon substrate 310 in a basic etching solution such astetramethylammonium hydroxide (TMHA), potassium hydroxide (KOH), orsodium hydroxide (NaOH), etc., to which surfactant such as isopropylalcohol (IPA), isopropyl ethanol (IPE), etc., is added.

After forming the texturing structure, thermal oxide layers 321 and 323for surface passivation are formed on the front and rear surfaces of thesilicon substrate 310. The passivation layers 321 and 323 have a role ofstabilizing and protecting the surface and minimizing surfacerecombination of electron-hole pairs to increase efficiency of the solarcell.

These passivation layers 321 and 323 may be thermal oxide such assilicon oxide (SiO₂), etc., formed by a rapid thermal oxidation (RTO)scheme performed inside a furnace for rapid thermal processing (RTP), asdescribed above. Also, the passivation layers 321 and 323 may be formedby a sputtering method using the silicon oxide (SiO₂) as a targetmaterial.

After forming the passivation layers 321 and 323, a pattern fordiffusing an n-type material is formed on the rear surface of thesilicon substrate 310, as shown in FIG. 12.

The formation of the pattern is to partially remove the oxidation layer323 already formed on the rear surface of the silicon substrate 310,thereby making it possible to diffuse an n-type material through theremoved portion.

A process forming the pattern by partially removing the oxide layer 323may be performed by irradiating laser light. As a light source of theirradiated laser light, various light sources may be used; by way ofexample, a green laser source with a wavelength of about 532 nm, anNd/YAG laser source with a wavelength of about 1064 nm, etc., may beused.

After partially removing the oxide layer 323, an n-type material 330 isdiffused into portions at which the oxide layer 323 is removed, as shownin FIG. 13.

As a method diffusing the n-type material 33, a thermal diffusionmethod, etc., may be used. By way of example, a method performing dopingby inserting the p-type silicon substrate 310 into a high-temperaturefurnace and flowing the n-type material (for example, POCl₃) into aninside of the furnace may be used.

After diffusing the n-type material 330, the oxide layer 323 is removedin order to form a p-type semiconductor region in a region except forthe region into which the n-type material is diffused, as shown in FIG.14. At this time, a certain amount of the oxidation layer 323 is left ina region close to the region into which the n-type material 330 isdiffused in order to insulate between the region into which the n-typematerial 330 is diffused and the p-type semiconductor region.

At this time, the oxide layer 323 may also be removed by irradiatinggreen laser light with the wavelength of about 532 nm, Nd/YAG laserlight with the wavelength of about 1064 nm, etc.

The oxide layer 323 left in order to isolate the region into which then-type material is diffused and the p-type semiconductor region mayperform a function of a rear passivation layer of the back contact solarcell. That is, the remaining oxide layer 323 protects the rear surfaceof the silicon substrate 310 as well as prevents rear surfacerecombination of electron-hole pairs so as to be able to contribute toimprovement in efficiency of the solar cell.

After removing all oxide layer 323 except for a certain amount of theoxide layer 323 for isolating the region into which the n-type material330 is diffused and the p-type semiconductor region among the oxidelayer 323 formed on the rear surface of the silicon substrate 310, thep-type semiconductor region 340 is formed by printing an aluminum (Al)metal, etc. in a region except for the region into which the n-typematerial is diffused, as shown in FIG. 15. The printing of the aluminum(Al) metal, etc., for forming the p-type semiconductor region 340 may beperformed by a screen printing method, etc., which is a well-knownprinting method.

In fabrication of the back contact solar cell of the present invention,since the removal of oxide layer 323 required for diffusing the n-typematerial 330 and the removal of the oxide layer 323 required for formingthe p-type semiconductor region 340 are performed by irradiating thelaser light, it is possible to omit a photolithography processrequisitely used to remove the oxide layer on the rear surface of theconventional back contact solar cell. Since the photolithographyprocess, which is a complicated and expensive process, may be omitted,it is possible to simplify the fabrication process of the back contactsolar cell and also reduce fabrication costs.

After forming the p-type semiconductor region 340, an anti-reflectioncoating 370 is formed on an upper surface of the oxide layer 321 formedon the front surface of the p-type silicon substrate 310, as shown inFIG. 16. The anti-reflection coating 370 may be deposited using adeposition method such as a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or a spin coating method, etc., andbe made of a material such as silicon nitride (SiN_(x)) or titaniumdioxide (TiO₂), etc. This anti-reflection coating 370 may have afunction of minimizing reflectance of the solar cell and at the sametime, perform a function of a passivation layer. Accordingly, defects ofthe back contact solar cell are minimized and recombination ofelectron-hole pairs is further reduced, so that the efficiency of thesolar cell may be improved.

After forming the anti-reflection coating 370, electrodes are printed oneach of the regions into which the n-type material is diffused and thep-type semiconductor region 340 to form rear electrodes 390, therebycompleting the back contact solar cell, as shown in FIG. 17. As theelectrode 390, a metal with a high conductivity such as silver (Ag),etc., may be used.

Although the present invention has been illustrated with regard tospecific details such as specific components, etc, limited embodiments,and drawings, the specific details such as specific components, etc, thelimited embodiments, and the drawings are only provided in order toassist overall understanding of the present invention. The preventinvention is not limited to the above embodiment, but may be variouslymodified and altered by those skilled in the art.

Therefore, a technical idea of the present invention is not limited tothe above-mentioned embodiment and claims described below andequivalents thereof are within a scope of the technical idea of thepresent invention.

According to the fabrication method of the back contact solar cell ofthe present invention, it is possible to simplify a fabrication process,reduce fabrication time, and reduce fabrication costs by using the lasermethod instead of photolithography in forming the pattern for formationof the p-type semiconductor region and the n-type semiconductor region.

Also, it is possible to obtain effects such as a rise in efficiency dueto minimization of the shadowing effect, which is an original advantageof the back contact solar cell, a reduction in recombination ofelectron-hole pairs due to rear surface passivation, etc.

What is claimed is:
 1. A method of making a back contact solar cell, themethod comprising: forming an oxide layer on a rear surface of asemiconductor substrate; forming a first conductive type semiconductorregion by doping a first dopant material of a first conductive type;forming a second conductive type semiconductor region by doping a seconddopant material of a second conductive type; patterning the oxide layeron the rear surface of the semiconductor substrate to locally remove theoxide layer on the rear surface at a first location to form a firstexposed portion of the semiconductor substrate to form a first contactbetween the first conductive type semiconductor region and a firstelectrode, and at a second location to form a second exposed portion ofthe semiconductor substrate to form a second contact between the secondconductive type semiconductor region and a second electrode; and formingthe first electrode at the first exposed portion and the secondelectrode at the second exposed portion.
 2. The method according toclaim 1, wherein the first conductive type is an n-type, and wherein, inthe forming the first conductive type semiconductor region, POCl₃ isused as a source of the first dopant material.
 3. The method accordingto claim 1, further comprising: forming an oxide layer on a frontsurface of the semiconductor substrate.
 4. The method according to claim1, wherein the patterning is performed using an irradiated laser light.5. The method according to claim 1, wherein the first dopant material isan n-type material and the second dopant material is a p-type material.6. The method according to claim 1, wherein the forming of oxide layeris performed as one of a rapid thermal oxidation (RTO) method performedinside a furnace for rapid thermal processing (RTP); and a sputteringmethod using silicon oxide as a target material.
 7. The method accordingto claim 1, wherein a portion of the oxide layer remains on the rearsurface of the semiconductor substrate.
 8. The method according to claim1, wherein the first conductive type semiconductor region and the secondconductive type semiconductor region is disposed with an interval on therear surface of the semiconductor substrate.
 9. The method according toclaim 8, wherein the rear surface of the semiconductor substrate has atexturing structure at the interval between the first conductive typesemiconductor region and the second conductive type semiconductorregion.
 10. The method according to claim 9, wherein the rear surface ofthe semiconductor substrate further has a portion having a flat surface,and the flat surface is between the first contact and the texturingstructure, the second contact and the texturing structure, or both. 11.The method according to claim 10, wherein a width of a portion of thefirst electrode is greater than a width of the first conductive typesemiconductor region, or a width of a portion of the second electrode isgreater than a width of the second conductive type semiconductor region.12. The method according to claim 9, wherein the rear surface of thesemiconductor substrate has the texturing structure locally tocorrespond only to the interval between the first conductive typesemiconductor region and the second conductive type semiconductorregion.
 13. The method according to claim 9, wherein the texturingstructure is not formed on at portions of the rear surface of thesemiconductor substrate corresponding to the first conductive typesemiconductor region and the second conductive type semiconductorregion.
 14. The method according to claim 1, wherein the oxide layer isinterposed between the first conductive type semiconductor region andthe second conductive type semiconductor region.
 15. The methodaccording to claim 9, wherein the oxide layer is formed on the rearsurface of the semiconductor substrate at an interval between the firstconductive type semiconductor region and the second conductive typesemiconductor region, and the oxide layer has a corresponding texturingstructure to the texturing structure of the semiconductor substrate. 16.A method of making a back contact solar cell, the method comprising:forming an oxide layer on a rear surface of a semiconductor substrate;removing at least one portion of the oxide layer by using a laser toexpose the semiconductor substrate; providing POCl₃ to the exposedportion of the semiconductor substrate; providing a first dopantmaterial to a first portion on the semiconductor substrate; providing asecond dopant material to a second portion on the semiconductorsubstrate; and forming a first electrode on the first portion and asecond electrode on the second portion.
 17. The method of claim 16,further comprising forming a first conductive type semiconductor regionon the first portion and forming a second conductive type semiconductorregion on the second portion.
 18. The method of claim 17, wherein theforming of the first conductive type semiconductor region and theforming of the second conductive type semiconductor region are performedby thermally diffusing the first and second dopant materialsrespectively into the first and second conductive type semiconductorregions.
 19. The method of claim 17, wherein the first conductive typesemiconductor region is an n-type and the second conductive typesemiconductor region is a p-type.
 20. The method according to claim 16,wherein a portion of the oxide layer remains on the rear surface of thesemiconductor substrate.